1. Field of the Invention
The invention relates to an opto-electronic distance measuring device with an intensity modulated measurement light beam based on the principle of phase delay measurement
2. Description of the Prior Art
Such opto-electronic distance measuring devices are suitable for measurement on technical surfaces, that is, they can be used without the use of defined targets. For determining the distance from a laser diode to an object being measured and from the object to the photodiode, an intensity of a measurement light beam of the laser diode is, as a rule, modulated, i.e., a measurement signal with a high measurement frequency heterodynes the measurement light beam. A phase delay process, in which a periodic, intensity-modulated emitted laser radiation is used, is now used in the majority of the current precise opto-electronic distance measuring devices. The distance is obtained from the measurement of the phase difference relative to two detected periodic phase delay signals along the measurement path or a reference path, which is retained even after downmixing to a lower intermediate frequency. The measurement of distance ranges up to several 100s of meters with precisions of a few millimeters is of considerable importance to numerous applications, in particular in the construction industry, trades and in plant engineering. In this range of requirements with highly precise measurements in a wide range of distances, the elimination or suppression of sources of measurement errors in opto-electronic distance measurement is necessary, especially the suppression of interfering low and high frequency interference modulations in the measurement light beam. Particularly in the case of very high measurement frequencies fM of 1 GHz, for example, the described interference modulation components cannot be avoided or can be avoided only at great expense. In addition, the interference modulation components are dependent on and aging, so that an initial calibration and the use of correction tables for troubleshooting is inadequate.
According to DE 10006493, several different measurement frequencies are used, in real situations more than two, with high measurement frequencies of, for example, 1 GHz, for the clear determination of distance ranges greater than 100 m, for example.
EP 601847B1 discloses a laser distance measuring device based on linear laser light frequency modulation (laser frequency chip), wherein for elimination of non-linearities by photonic mixing of laser chip signals using a laser local oscillator signal, an electrical control signal is generated, which oscillates with a frequency difference between both laser signals. Using control signals from a microcontroller, a side band is suppressed by means of quadriphase modulation in the spectrum of the electrical control signal. In addition, the microcontroller is used for generating a correction signal in order to compensate in advance measured non-linearities in the laser frequency chip, and for generation of a temporarily stored tracking/hold signal in order to keep a measured error signal close to zero.
According to EP 439011, the laser diodes are modulated sequentially using four phase shifted high frequency oscillator signals offset by 90 degrees. In a receiver, the respective reception signals are multiplied by the correspondingly sent oscillator signals, whereby four identical signals are generated. Using this system comprised of four equations, the interference direct components and signal amplitudes can be eliminated, and the phase shift or the signal delay and, thus, the distance can be determined. An approximation filter disposed downstream of the receiver is controlled by the microcontroller relative to bandwidth. In addition, using the driving circuit of the transmitter, the laser diode current and, accordingly, the emitted light output is controlled. For the purpose of regulating the light output via the d.c. operating point of the laser diode, the laser diode has a monitor photodiode in its housing, using which the mean output light power of the laser diode is measured and controlled by an analog PI regulator. Control of the d.c. operating point of the laser diode or of the approximation filter is not appropriate for purposes of interference suppression.
The object of the invention is to provide a device and a method for reducing the measurement errors occurring due to interference modulation components in the measurement light beam of an opto-electronic distance measuring device.